Mercruiser 2bbl carb tuning

[DaveN]

I've got a 1997 4. 3L sea ray, and we're having a hard time starting it. Once it's warm, it starts and runs fine with no issues. Cold starts are a complete nightmare. It cranks and cranks, and doesn't even act like it wants to start... ... sometimes. Othertimes you give it one pump on the throttle and it fires right up. Warm starts are fine with no problem. It's hard to troubleshoot because it doesn't do it all the time, but when it does do it it's typically after not running for at least 24 hours.



I've found recently that when we're in the crank forever mode, a teeny shot of starter fluid will get it to fire and run perfectly. The choke looks like it's working, closing when I first start it then opening after it warms up. I don't know if it's tuned properly though. So it's telling me that I have an intermittent fuel problem.



And then, to complicate things more, occasionally when were in the crank forever mode, and I get angry and start pumping the throttle, it floods and leaves me sitting for even longer. So I'm kind of stumped, sometimes I have too much fuel, sometimes I don't have enough.



I'm considering a rebuild or replacement, but I want to make sure it's tuned properly and/or cover the basics first. I've found out that it's the Mercarb 2bbl carb on this one, I was wondering if anyone had any expereince with these carbs or this problem.



Thanks



Dave

 

[TowPro]

Is this a GM motor? Check the rotor, take it off the distributer, turn it upside down. Look for a black spot around the center like a small burn mark. I have seen these burn through the top of the rotor and short out to the distributer shaft.

 

[DaveN]

It is is a GM 4. 3, I'll check that out. But if it had shorted, wouldn't that cause a no run, or would it run sometimes?

 

[midlife crisis]

Dave- They can be finicky to say the least. I always found they would need just one pump of the throttle to set the choke and to also give one squirt of fuel. Anymore than that and chances of flooding are high.

One other trick that always yielded results was a flushing of the fuel circuits themselves. With the flame arrestor off, run up the rpms to 3000 or so and snuff the carb with your hand- as she comes down in rpm, work the carb's throttle arm quickly up & down with your hand. Pull your hand away so the engine will keep running- you don't want it to stall. 2-3 times with a few seconds in between seem to help smooth a pesky carb. Remove the control cable from the carb will help so you aren't fighting it.

As I am sure you do indeed have a fuel problem- the tune-up cant hurt, especially if the plugs were fouled during winterizing.

Go with AC plugs. I never did like Champions-

Good Luck-

 

[HOBrian]

Check your fuel filter. Since you are having trouble with it starting it might be a bad fuel filter. If this is not it I would go with what Drew said. (MC)

 

[TowPro]

It is is a GM 4. 3, I'll check that out. But if it had shorted, wouldn't that cause a no run, or would it run sometimes?

Here is what I am going on. When the spark is able to jump through the rotor to ground (the distributer shaft), it leaves a carbon trace. This trace will allow it the spark to jump easer every time.



Now if you have ever watched a gas engine run on a scope, there are times that spark plugs take more voltage to fire. Startup can be one of those times. Add the extra voltage requirements to the conditions a boat engine is always in (the water) and you can have the spark decided its easier to jump across the carbon track in the rotor then the spark plugs.



I have seen this several times in the past. when I would get sent out on a road call and it was a GM, I always took a rotor with me. Symptoms I have seen caused by this problem were no cold start, no hot start and missing under load.

they used to give you a dielectric grease to put under the rotor when you install it. Also, while your in there, if it has a mechanical advance, take a close look at it. Is it covered with rust? Does it still move? Clean it off, check the bushings where the weights pivot. Put one drop of oil on each pivot point.



My all means, it could very well be the carb, but you posted when its cold the choke closes, and it opens some when started which shows the choke pulloff is working. Also check that little fuel filter in the carb (if it has one). they used to clog real easy.



when you flood out a newer engine, unless raw fuel in running down into the engine, there is a good chance no spark can cause a flooded engine. An easy test I use is a sparkplug I made with a battery charger clamp mounted to it. Clip the clamp on ground, put the spark plug wire on the plug and crank the engine.

KEEP IN MIND YOU ARE IN A BOAT. GAS FUMES LIKE TO STAY IN THE HULL, DON'T BLOW UP YOUR BOAT.



Just remember, you only need 3 things to make a gas engine run, Gas, Spark and Compression (all at the correct times).

 

[Alan Reagan]

I had a Mercruiser that had the same problem. I pulled into a marina in St. Marks Florida, tied up and spent the night. The next morning, no start. The marina operator told me to push the throttle all the way forward and then all the way back and then back to neutral. I did it and the engine cranked. He said it was the microswitch on the top rear of the engine. When the lever is pushed forward or back to go into gear, it momentarily kills the engine to allow the shift. So then engine dies and restarts so fast that you don't notice it. I had thought up to that point that it was the carb but it wasn't. Or at least I never had the problem again after replacing the switch.



Being in the Coast Guard, I have passed that trick to more than one boater whose boat wouldn't start. Many times, it has worked. Hope this helps you.

 

[TORQUE THIS]

Here is a good site for info... Mercruiser engine and sterndrive help and assistance

Also, free to join here also... Club Sea Ray